I talked to my hardware expert guy when I recognized there´s Intel i9, x299 and z370 now.
I had to explain XITE-1 DSP farm and SCOPE 5.1 show issues since z170 was introduced and z97 is the last working w/ full bandwidth on PCIe bus.

His answer was there were several changes in adress reservations for M.2 being introduced w/ z170 / x99 / z270 / x299 and he expects nothing will be better w/ z370 for SCOPE and XITE-1 PCIe card data thruput until S|C comes w/ a solution.
I think it makes sense ´cause I remember z97 handled only 1 M.2, some z170 mobos handled only 1 M.2 too but some other already handled 2, while all z270 mobos handled 2 of these always.
More M.2 is the only very obvious hardware change in z170 and z270 mobos.

Ryzen chipsets might not be better.
I guess AMD want´s to adress many M.2 slots as well and there aren´t so many different ways eventually.

It might be some ressource sharing issue between M.2 and PCIe slots and AFAIK now, it makes no difference if there is any M.2 media inserted into a slot or not as also I´m to aware of disabeling M.2 slots completely.

Can be mobo design/architecture plays a role too and there might be some where the issue is less obvious tham w/ others.

If S|C released a solution I'm sure people would buy - On Z97 via PCI-e x 1 slot it is just 1 x PCI-e lane thru an x1 slot - which would make Xite quite slow on only 1 x PCI-e lane - I'm not saying it's not good enough -

What is the best solution here moving forwards?

People with older Z97 or older boards can have good use still - Someone got Xite working on Z270?

PCIe 1x is more than fast enough for the XITE's use.
PCIe 4x works exactly the same as 1x, if a 1x card is in the 4x slot.
the only disadvantage to the 4x slot is that it's easier to misalign the card, but misaligning the card is still pretty hard to do.

it looks like you will have to do the testing. you can't just wait for someone else to waste money...

I use PCI 3 (blue) and a PCI-e 4X slot for my S|C Connector card.
Fits fine, works fine, and due to the chipset/CPU Design Audio doesn’t go to the PCH but direct to CPU.
H97/Z97 chips are a unique/rare design stage where M.2s were @ 768MBps (PCI 2x speeds) so Phison E7 Controller Based devices are perfect.
We run into problems with boards that have more PCI Lanes that are unnecessary, that run at 1800MBps or higher.
These can be disabled in device manager though as they are pretty much useless except to load samples, rather than stream them.
STS or NNXT, Omnisphere, Love M.2s. Kontakt.....will overheat your PCI 4x M.2 making it throttle down to PCI 2X speeds anyway. Right back in the safe zone

Notice my E7 NVMe has a side blower fan to keep things stable.
You can also see the blue level 3 slot, and notice it’s a 4x.

H97/Z97 chips are a unique/rare design stage where M.2s were @ 768MBps (PCI 2x speeds) so Phison E7 Controller Based devices are perfect.
We run into problems with boards that have more PCI Lanes ... that run at 1800MBps or higher.
These can be disabled in device manager though ...

Interesting observation !

Disable dedicated (for the user obsolete) lanes completely in device manager is new to me.
How to find out what we SCOPE users w/ XITE-1 don´t really need and when disabled, how to be sure about other apps and hardware won´t suffer from disabled lanes ?

And,- does lane-disabling in device manager work w/ all mainboards/chipsets (and BIOSes,- if BIOS matters at all) ?

Not sure but noticed the Z270/i7 7700k running FL Studio 12.5 for Junior had options in the BIOS, which means they would likely be disabled in the Device Manager too.

I like having the “slower” 2x PCI Speed devices because they’re larger and cheaper and there’s no difference in transaction speed or access.
Everybody else buys 4x making my Phison E7 Based 2x devices cheap.
Since they load samples instead of streaming they barely get warm.

I like being behind the times.
I bet Scope 7 will also like Windows 8.1 and H97.
I’m perfectly happy with Z/H97 and Scope 5 though just in case.

I wish Rack mount optimized boards for Ryzen in 2018 show up.
If not I’ll stay 4 years behind the curve.
Happier making music instead of fixing things that don’t need fixing...